Ceramic matrix composites (CMCs) have attracted significant interest for various aircraft and power generation applications due to their high toughnesses and their ability to avoid catastrophic failure when their maximum load carrying capability has been exceeded. CMCs are also much lighter and have higher temperature capabilities than the superalloys currently used for many gas turbine engine components, making CMCs particularly attractive for aerospace applications. CMCs generally comprise three components: a ceramic matrix, reinforcing fibers embedded within the ceramic matrix, and an interfacial coating on the reinforcing fibers. The high toughness of CMCs is due to the weak or limited bonding that occurs between the fibers and the surrounding matrix, which allows the fibers to pull out of the matrix instead of causing a catastrophic failure. Interfacial coatings are often applied to such fibers to control the chemical and mechanical interfacial bonding between the fibers and the surrounding matrix, ensuring the bond therebetween remains weak, thereby improving the composite's mechanical properties (i.e., strength, toughness, etc.). These interfacial coatings can also protect the fibers from mechanical damage during handling and processing, and they may limit oxidation in the composite.
While boron carbide coatings have been utilized as interfacial fiber coatings for many applications, boron carbide coatings do not have the high temperature capability that boron nitride coatings do. As such, boron nitride coatings are more desirable for many applications, such as for divergent seals in gas turbine engines, which operate in aqueous environments at temperatures as high as about 1200° C.
While many methods exist for coating fibers with boron nitride, they all have drawbacks. Therefore, it would be desirable to have improved methods for coating fibers with boron nitride. It would also be desirable to have improved methods for removing the sizing from such fibers before applying the boron nitride coating thereon. It would be further desirable to have methods for nitriding the surface of the fibers before applying the boron nitride coating thereon, so as to prevent boron from diffusing into the fibers.